An active matrix type liquid crystal display device is provided with an ESD protection circuit to protect the device from harmful electrostatic discharge which amounts to several tens of thousands of volts. As a conventional ESD protection circuit, a diode using a saturation region operation of a TFT (thin film transistor) is used.
FIG. 1 shows a conventional ESD protection circuit, FIG. 1(a) shows an equivalent circuit, and FIG. 1(b) shows wiring patterns in an ESD protection circuit.
In FIG. 1(b), the ESD protection circuit uses a TFT 3 comprising a source bus 1 and a gate bus layer 2 provided on the source bus 1. More specifically, a diode using the saturation region operation of this TFT 3 is used as the ESD protection circuit.
FIG. 2 is a sectional view along II-II line in FIG. 1(b). In FIG. 2, a light shield film 12 for preventing the light from directly entering the TFT is provided on a glass substrate 10, which is an insulating transparent substrate. This light shield film 12 is formed in a region on the glass substrate corresponding to the region including the region (gap) between a source electrode and a drain electrode. A silicon oxide film 11, which is an interlayer insulating film, is formed on the glass substrate 10 on which the light shield film 12 is formed.
A source electrode (source bus) and a drain electrode are formed on the silicon oxide film 11. These source electrode and drain electrode each have a double-layered structure consisting of an ITO film 13 and a metal film 14 formed on the ITO film 13, both of which are transparent electrodes. A gap is formed between the source electrode and the drain electrode, and an a-Si film 15, which is a semiconductor film, is formed in the gap and on the source electrode and the drain electrode around the gap.
A silicon nitride film 16, which is a gate insulating film, is formed on the a-Si film 15. A contact-hole 16a is formed in this silicon nitride film 16. A gate bus layer 17 is formed in the region including the gap of the silicon nitride film 16. A film 17a which is the same layer as the gate bus layer 17 formed in the contact-hole 16a is connected to the source electrode.
Such a structure requires one ESD protection circuit TFT per one diode, and therefore as shown in FIG. 1(b), at least two TFTs per one bus line are required. Furthermore, since this diode uses a TFT, it has a complicated structure and requires a relatively wide space. Moreover, during the operation of the TFT in a saturation region, the current rises with respect to the voltage like a quadratic curve, and so it is not preferable from the standpoint of a diode characteristic.